In the sector of photographic reproduction photohardenable recording materials have been used, in which differences in tackiness of the exposed and unexposed regions of the photohardenable film have been utilized for image production.
Thus, for example, from the German patents 1 210 321, 1 904 058, 1 904 059 and 2 004 214, a reproduction process has become known in which a tacky photopolymerizable recording material, consisting of a film support and a photopolymerizable film, which contains at least one addition-polymerizable monomer and photopolymerization initiator, is hardened through imagewise exposure, whereby the exposed image portions lose their tackiness. The latent image is then made by applying suitable toner materials, which adhere only to the unexposed, tacky places, whereas, they can again be removed from the exposed, nontacky image portions after the application. Using this process, positive, possibly colored images of the original are obtained which are equal in appearance to pictures that were prepared with the use of printing inks. Therefore, the process has achieved considerable practical importance, especially in the printing industry, for testing color separations.
A not yet eliminated disadvantage of the process consists in that the achievable toned density is dependent on the toning temperature present, whereby especially strong fluctuations appear in the region of room temperature between 18.degree. C. and 35.degree. C. This temperature dependence of the toning process leads to a nonuniform covering of the unexposed regions with toner material, whereby a raising of the toning temperature produces an increased plasticity of the tacky, photohardenable film and therewith also an increased toner take-up.
Besides, the temperature dependence of the toning process can also lead to fog and spot formation as well as faulty colors. Especially in color testing processes, which must reproduce the color density and color shade of the production run exactly and reproducibly, such an impairment of the image-forming quality cannot be tolerated. The toning is usually carried out under given room conditions. A toning under constant climatic conditions requires expensive equipment, which is not practicable in practice, because a color testing process must be simple, fast in use and with a more favorable cost than the conventional proof copy method. From the German Offenlegungsschrift 2 364 179 and the German Offenlegungsschrift 3 012 841, it is known that the surface properties of relief printing forms can be improved through addition of compounds with cross-linking, multivalent metal cations and, in fact, above all can reduce the tackiness of the printing form.
From the German Offenlegungsschrift 2 364 179, it is known that multivalent metal salts are suitable for improving the adhesion between printing relief and substrate.
However, these patent applications do not disclose any photohardenable image-forming materials, which are suitable for the preparation of tonable image formations.
The problem of the present invention is to provide a positive working, tonable, photohardenable mixture, in which the temperature dependence of the toning process is reduced without impairment of other essential properties of the material, for example, the achievable toned density.
This problem was solved for the first time by a positive operating photohardenable mixture according to the characterizing portion of claim 1.